Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) such as gefitinib, erlotinib, and afatinib have dramatically improved outcomes in non-small cell lung cancer (NSCLC) patients with EGFR mutations. Unfortunately, acquired resistance develops after a median of 10-13 months and treatment toxicities (acneiform rash, diarrhea) can limit combination approaches to restore the sensitivity. The most commonly clinically observed mechanism of EGFR TKI resistance is the T790M mutation, found in 40-50% of erlotinib- or gefitinib- resistant tumors. Other T790M-independent resistance mechanisms include bypass of EGFR via HER-2 or MET, STAT3 or NF-B activation, or epithelial-to-mesenchymal-transition (EMT)-associated AXL signaling. There are no approved agents effective against EGFR with a T790M mutation. Developing effective and tolerable regimens for overcoming EGFR TKI resistance remains an important unmet need. Our preliminary studies have shown that ibrutinib, an inhibitor of Bruton's tyrosine kinase recently FDA approved for several B-cell malignancies, functions as an inhibitor of mutant EGFR and induces anticancer activity in EGFR-mutant NSCLC cells, including erlotinib-resistant cells harboring a T790M mutation, and also inhibits HER-2 signaling. This unexpected finding led us to develop a clinical trial to treat EGFR-mutant NSCLC with ibrutinib, which will begin accrual early next year. Moreover, ibrutinib was recently reported to be well tolerated in combination with other targeted therapeutics. These results provide proof-of-principle evidence of the feasibility of overcoming resistance to anti- EGFR therapy by repurposing FDA-approved drugs. Based on our preliminary data, we hypothesize that ibrutinib represents a novel EGFR inhibitor that may overcome TKI resistance through its activity against EGFR T790M and HER2, and through its combination with other targeted therapeutics. Because of ibrutinib's activity and favorable toxicity profile, it may be a useful backbone for combination regimens that simultaneously target multiple resistance mechanisms. Nevertheless, significant knowledge gaps remain regarding the activity of ibrutinib and optimal combination regimens to overcome resistance. To address these gaps we propose the following Specific Aims: 1) Determine the direct effect of ibrutinib on clinically relevant mutations in preclinical models, including T790M mutations and HER2 overexpressions; 2) Characterize the mechanisms of T790M- independent resistance to ibrutinib in preclinical models, and develop more effective combination regimens including those with STAT3/NF-B and AXL inhibitors; and 3) Test the efficacy of ibrutinib in EGFR-mutant NSCLC patients and, using specimens from this trial, investigate the determinants of ibrutinib response and resistance. This proposal has significant public health implications because overcoming EGFR TKI resistance will have a significant impact on clinical outcomes and quality of life of EGFR-mutant NSCLC patients; furthermore, by repurposing agents already in routine clinical use, it is poised for rapid clinical translation.